Norman McIlwain is an evangelical Christian but an opponent of the PST. He has written a book entitled, The Biblical Revelation of the Cross. It is available online. In an addendum to the book, he points out why the NT concept of the believer's union with Christ is of no help in justifying the PST.
1. It would involve Christ and the church sharing personal identity.
While a husband and wife may become "one flesh," that does not mean they become one person. They share many things but they don't share everything.
A husband takes on the wife’s financial debt and pays it off. In marriage, wealth and financial burdens can be shared, but acts of sin on the part of one committed without complicity on the part of the partner cannot implicate the partner in the guilt or cause the other to justly suffer for the offence. Husbands and wives are not made guilty for the sins of their partners. It matters not that the partner might be willing to suffer for the crime of the other. Although there can be a marriage of wills to share certain responsibilities and burdens, there can be no marriage of wills where there is no complicity in the committing of a crime. We are each held responsible for our own sins. So it is with Jesus. In marriage with the Church, there is no marriage of His will with the will of man regarding acts of sin – no complicity. Therefore there can be no imputation of guilt or sin.
[Moreover], to whom is the world in debt, because of sin? Against whom have all sinned? – As both Man and God, the Word made flesh is the One against whom all have sinned. Mankind is in debt to Christ. . . . So, why should the Groom suffer punishment for the unpaid debt of the bride, when He Himself is the bride’s creditor?
2. It would involve Christ ceasing to be righteous.
If the union means that man's sin is transferred to Christ and man is no longer guilty, then it ought to also mean that when Christ's righteousness is transferred to man, Christ is no longer righteous.
The idea that man’s sins can somehow be imputed to Christ does not work. Jesus does not consent to sin. The idea presented above suggests that by imputing our sins to Christ, we somehow are set free. Yet, Jesus retains His righteousness, though His righteousness is imputed to His followers. By the same token, we should retain our guilt and sin, though these be imputed to Christ. It simply does not make sense, nor can it be just.